The invention primarily relates to a hydraulic hybrid drivetrain. The invention further relates to a method of shutting down an engine of a hydraulic hybrid drivetrain and to a method of starting an engine of a hydraulic hybrid drivetrain using hydraulic energy.
An important percentage of a life of a working machine consists of being placed in an idling condition, without any request from the operator. In the idling condition, the working machine is stopped and an engine of the working machine is operating at a minimal speed. In the idling condition, the engine of the working machine, which may be an internal combustion engine, could be shut off in order to reduce fuel consumption of the working machine. To re-start the engine, known working machines use an electric motor, commonly referred to as a starter, to accelerate the engine to a minimum rate that enables the fuel injection and engine firing processes. Usually, vehicles that include an automatic start-stop functionality (which is typically based on the recognition of missing input from an operator and the idling condition being present for a predetermined amount of time) are equipped with an over-sized electric starter and main battery, due to the increased amount of engine start requests during the life of the vehicle.
It is therefore an object of the present invention to design a driveline capable of providing an energy-efficient start-stop functionality. It is another object of the present invention to devise energy efficient methods of shutting down an engine and of re-starting the engine.
These objects are solved by a series hydraulic hybrid driveline, by a method of shutting down an engine of a series hydraulic hybrid driveline, and by a method of restarting an engine of a series hydraulic hybrid driveline according to the independent claims. Special embodiments are described in the dependent claims.
Thus, a series hydraulic hybrid driveline is proposed, in particular for use in an automotive vehicle, the driveline comprising:                a hydraulic circuit comprising a first hydraulic displacement unit in fluid communication with a second hydraulic displacement unit, the first hydraulic displacement unit having a variable hydraulic displacement and the first hydraulic displacement unit being drivingly engaged with an internal combustion engine;        a hydraulic accumulator assembly selectively fluidly connected to the hydraulic circuit, the hydraulic accumulator assembly comprising a high pressure hydraulic accumulator and a low pressure hydraulic accumulator; and        a hydraulic actuator adapted to control the hydraulic displacement of the first hydraulic displacement unit, the hydraulic actuator being in fluid communication with the hydraulic accumulator assembly and/or with the hydraulic circuit.        
As the accumulator assembly is selectively fluidly connected to the hydraulic circuit comprising the first hydraulic displacement unit and as the first hydraulic displacement unit is drivingly engaged or selectively drivingly engaged with the internal combustion engine, hydraulic energy stored in the accumulator assembly may be used to accelerate the internal combustion engine through the first hydraulic displacement unit for starting the engine. Starting the engine through the first hydraulic displacement unit typically includes displacing hydraulic fluid from the high pressure accumulator to the low pressure accumulator through the first hydraulic displacement unit, thereby driving the first hydraulic displacement unit.
Furthermore, as the hydraulic actuator is in fluid communication with the hydraulic accumulator assembly and/or with the hydraulic circuit, hydraulic energy stored in the accumulator assembly and/or in the hydraulic circuit may be transmitted to the hydraulic actuator for controlling the hydraulic displacement of the first hydraulic displacement unit and for setting the hydraulic displacement to a desired value for a starting operation or for shutting down the engine. Controlling the displacement of the first hydraulic displacement unit may include at least one of increasing and decreasing the displacement. Controlling the displacement of the first hydraulic displacement unit may further include setting the displacement to zero displacement, for example when or right before shutting down the engine and/or when or right before fluidly disconnecting the accumulator assembly from the hydraulic circuit.
The steps of using the hydraulic actuator to set the hydraulic displacement of the first hydraulic displacement unit to a desired value and of transmitting hydraulic energy stored in the accumulator assembly to the first hydraulic displacement unit for starting the engine may be carried out using no electric energy or only a minimum amount of electric energy, for example for switching one or more electrically controlled valves.
The hydraulic circuit usually comprises a first main fluid line fluidly connecting or selectively fluidly connecting a first fluid port of the first hydraulic displacement unit to a first fluid port of the second hydraulic displacement unit, and a second main fluid line fluidly connecting or selectively fluidly connecting a second fluid port of the first hydraulic displacement unit to a second fluid port of the second hydraulic displacement unit. Each of the high pressure accumulator and the low pressure accumulator may then be selectively fluidly connected to the first main fluid line and to the second main fluid line. A minimum hydraulic or hydrostatic pressure in the hydraulic circuit may be at least 10 bar or at least 20 bar.
The hydraulic actuator may include one or more actuators configured to convert hydraulic or hydrostatic energy to a mechanical force or to mechanical movement. For example, the hydraulic actuator may include but is not limited to a hydraulic piston or a hydraulic motor.
The first hydraulic displacement unit may include a hydraulic, typically hydrostatic pump. For example, the first hydraulic displacement unit may be a hydrostatic radial piston pump or a hydrostatic axial piston pump having a moveable swashplate. The hydraulic actuator may then be mechanically coupled to the swashplate for moving the swashplate and for controlling a position or a swivel angle of the swashplate.
The second hydraulic displacement unit may include a hydraulic, typically hydrostatic motor. Like the first hydraulic displacement unit, the second hydraulic displacement unit may have a variable hydraulic displacement. The second hydraulic displacement unit may be a hydrostatic radial piston motor or a hydrostatic axial piston motor having a moveable swashplate. Usually, the second hydraulic displacement unit is drivingly engaged or selectively drivingly engaged with a vehicle output. The vehicle output may include at least one of a drive shaft, a final drive, a vehicle axle and wheels, for example.
The system typically comprises an electronic control unit. The control unit may be configured to control one or more valves and/or electronic actuators for at least one of: selectively fluidly connecting the accumulator assembly to and disconnecting the accumulator assembly from the hydraulic circuit, selectively fluidly connecting/disconnecting the accumulator assembly and/or the hydraulic circuit to/from the hydraulic actuator, controlling a hydraulic pressure applied to the hydraulic actuator through the accumulator assembly and/or through the hydraulic circuit. Within the scope of this document the formulation “at least one of x1, . . . , xn” may include any subset of x1, . . . , xn, including the complete set. The control unit may further be configured to start the engine and to shut down the engine.
When or right before shutting down the engine, a state of charge of the accumulator assembly may be checked in order to make sure that enough hydraulic energy is stored in the accumulator assembly for re-starting the engine. To that end, the system may be equipped with one or more pressure sensors adapted to determine a state of charge of the accumulator assembly. For example, the system may comprise a first pressure sensor for determining a hydraulic pressure in the high pressure accumulator and/or a second pressure sensor for determining a hydraulic pressure in the low pressure accumulator. The state of charge of the accumulator assembly may include at least one of the hydraulic pressure in the high pressure accumulator, the hydraulic pressure in the low pressure accumulator, and a pressure difference between the hydraulic pressure in the high pressure accumulator and the hydraulic pressure in the low pressure accumulator. The control unit may be configured to control the pressure sensor(s). For example, the control unit may be configured to at least one of command the pressure sensor(s) to perform a pressure measurement, receive the result of a pressure measurement performed by the pressure sensor(s), and process the result of the pressure measurement to determine the state of charge of the accumulator assembly.
The hydraulic actuator may be in fluid communication with the hydraulic accumulator assembly and with the hydraulic circuit through a pilot pressure portion, the pilot pressure portion comprising shuttle valves adapted to select a greatest hydraulic pressure between i) a greatest hydraulic pressure in the hydraulic circuit and ii) a greatest hydraulic pressure in the hydraulic accumulator assembly. The greatest pressure selected by the shuttle valves may then be used as a pilot pressure for controlling the hydraulic actuator. For example, the greatest pressure selected by the shuttle valves may be applied or selectively applied to the hydraulic actuator for controlling the displacement of the first hydraulic displacement unit.
A shuttle valve typically includes a hollow body, such as a pipe, that has two inlets and one outlet. A blocking element may be freely moveable within the hollow body to selectively block one of the two inlets, thereby allowing a flow of hydraulic fluid between the inlet that is not blocked and the outlet. For example, when a first hydraulic pressure is applied to the first inlet and a second hydraulic pressure is applied to the second inlet, the first hydraulic pressure being larger than the second hydraulic pressure, the blocking element is pushed toward the second inlet, thereby blocking the second inlet. In this way, the first inlet is fluidly connected to the outlet and the hydraulic pressure at the outlet is equal to the first hydraulic pressure applied to the first inlet.
Optionally, a shuttle valve may be replaced by an arrangement including at least two check valves as is readily apparent to a skilled person. For example, such an arrangement may include a first check valve providing fluid communication between a first (inlet) fluid port and an outlet fluid port, and a second check valve providing fluid communication between a second (inlet) fluid port and the outlet fluid port. In this arrangement, the first check valve may be configured to allow a flow of fluid from the first fluid port to the outlet fluid port and to block a flow of fluid from the outlet fluid port to the first fluid port, and the second check valve may be configured to allow a flow of fluid from the first fluid port to the outlet fluid port and to block a flow of fluid from the outlet fluid port to the first fluid port. In this manner, the greatest of the hydraulic pressures applied to the inlet fluid ports is selected at the outlet fluid port.
The pilot pressure portion may further comprise a pressure reducing valve adapted to reduce the greatest pressure selected by the shuttle valves to a preferably constant pilot pressure. The pressure reducing valve may be fluidly connected or selectively fluidly connected to the hydraulic actuator, for example through one or more shut-off valves, for providing or for selectively providing a reduced pilot pressure to the hydraulic actuator. Limiting the pilot pressure in this manner may prevent damaging the hydraulic actuator. Furthermore, providing a constant pilot pressure to the hydraulic actuator may facilitate controlling the hydraulic actuator and, thus, controlling the hydraulic displacement of the first hydraulic displacement unit.
Specifically, the pilot pressure portion may comprise a first shuttle valve adapted to select a greater hydraulic pressure between the hydraulic pressure in the high pressure hydraulic accumulator and in the low pressure hydraulic accumulator. The pressure selected by the first shuttle valve may be applied or selectively applied to the hydraulic actuator, optionally after passing the above described pressure reducing valve. As explained above, the first shuttle valve may optionally be replaced by a set of at least two check valves.
Alternatively, the pilot pressure portion may comprise a second shuttle valve adapted to select a greater hydraulic pressure between the hydraulic pressure in the first main fluid line and in the second main fluid line. The pressure selected by the second shuttle valve may be applied or selectively applied to the hydraulic actuator, optionally after passing the above described pressure reducing valve. As explained above, the second shuttle valve may optionally be replaced by a set of at least two check valves.
Further, it is conceivable that the pilot pressure portion comprises both the above described first shuttle valve, the above described second shuttle valve and, additionally, a third shuttle valve, wherein the third shuttle valve is adapted to select a greater hydraulic pressure between the hydraulic pressure selected by the first shuttle valve and the hydraulic pressure selected by the second shuttle valve. That is, the pressure selected by the third shuttle valve is the greatest hydraulic pressure of the hydraulic pressures in the accumulators and in the hydraulic circuit. The pressure selected by this third shuttle valve may then be applied or selectively applied to the hydraulic actuator, optionally after passing the above described pressure reducing valve. Again, the third shuttle valve may optionally be replaced by a set of at least two check valves.
The proposed driveline may comprise a charge pump drivingly engaged with the internal combustion engine. The charge pump may be in fluid communication with the hydraulic actuator for providing or for selectively providing a pilot pressure to the hydraulic actuator when the internal combustion engine is driving the charge pump. The charge pump may be fed by a fluid reservoir. The fluid reservoir may be at atmospheric pressure.
The driveline may comprise a pair of isolation valves adapted to selectively fluidly isolate or disconnect the second hydraulic displacement unit from the first hydraulic displacement unit and/or to selectively fluidly isolate the second hydraulic displacement unit from the hydraulic accumulator assembly, in particular when the hydraulic accumulator assembly is fluidly connected to the hydraulic circuit. Fluidly isolating the second hydraulic displacement unit from the first hydraulic displacement unit and/or from the accumulators may be useful when starting the engine in order to disengage the vehicle output. Alternatively, the hydraulic displacement of the second hydraulic displacement unit could be set to zero or the second hydraulic displacement unit could be disengaged from the vehicle output using a clutch, for example.
Further, a method of shutting down an internal combustion engine of a series hydraulic hybrid driveline is proposed. The method may be carried out using the hydraulic hybrid driveline described above. The method comprises the steps of:                when, for a predetermined period of time, no input command is provided through at least one of one or more input devices, determining a state of charge of a hydraulic accumulator assembly; and        if the determined state of charge is below a threshold state of charge:                    charging the hydraulic accumulator assembly until the state of charge of the hydraulic accumulator assembly is equal to or larger than the threshold state of charge, and then shutting down the engine;                        if the determined state of charge is equal to or above the threshold state of charge:                    shutting down the engine.                        
Preferably, the threshold state of charge is the minimum state of charge or the minimum hydraulic energy that is needed for re-starting the engine using the accumulators as explained above. The method may be carried out using the above described hydraulic control unit and pressure sensor(s). Charging the accumulators may include using the engine to drive the first hydraulic displacement unit drivingly engaged with the engine, so that the first hydraulic displacement unit displaces hydraulic fluid from the low pressure accumulator to the high pressure accumulator, thereby increasing a pressure gradient between the accumulators.
Finally, a method of starting an internal combustion engine of a series hydraulic hybrid driveline is proposed. The method may be carried out using the series hydraulic hybrid driveline described above. The method comprises at least the following steps:                fluidly connecting a high pressure hydraulic accumulator and a low pressure hydraulic accumulator to a first hydraulic displacement unit drivingly engaged with the engine; and        accelerating the internal combustion engine through the first hydraulic displacement unit by displacing hydraulic fluid from the high pressure accumulator to the low pressure accumulator through the first hydraulic displacement unit.        
Fluidly connecting the high pressure accumulator and the low pressure accumulator to the first hydraulic displacement unit usually includes fluidly connecting the high pressure accumulator to a first fluid port of the first hydraulic displacement unit and fluidly connecting the low pressure accumulator to a second fluid port of the first hydraulic displacement unit, or vice versa, typically by actuating one or more valves and/or by actuating one or more electric actuators.
The method may further comprise the steps of:                fluidly connecting the high pressure hydraulic accumulator to a hydraulic actuator adapted to control the hydraulic displacement of the first hydraulic displacement unit; and        varying the hydraulic displacement of the first hydraulic displacement unit through the hydraulic actuator by displacing hydraulic fluid from the high pressure hydraulic accumulator to the hydraulic actuator.        
The step of varying the hydraulic displacement of the first hydraulic displacement unit may include setting the hydraulic displacement of the first hydraulic displacement unit to a non-zero displacement before fluidly connecting the high pressure accumulator and the low pressure accumulator to the first hydraulic displacement unit.
When the ICE has been accelerated, for example when the ICE has reached an idling speed, the hydraulic actuator may be used to set the hydraulic displacement of the first hydraulic displacement unit to zero displacement so that the accumulators may subsequently be fluidly disconnected from the hydraulic circuit in a controlled manner and without causing cavitation in the first hydraulic displacement unit. Setting the hydraulic displacement of the first hydraulic displacement unit to zero may be carried out by using a charge pump drivingly engaged with the internal combustion engine and in fluid communication with the hydraulic actuator to apply a hydraulic pilot pressure to the hydraulic actuator.
The method may be initiated automatically once the state of charge of an electric energy storage device, such as a battery, falls below a threshold state of charge. The threshold state of charge may be the amount of electric charge and/or of electric energy required for fluidly connecting the high pressure hydraulic accumulator and the low pressure hydraulic accumulator to the first hydraulic displacement unit and/or for fluidly connecting the high pressure hydraulic accumulator to the hydraulic actuator, for example by actuating one or more electric valves and/or one or more electric actuators. When the engine has been started, the electric energy storage device may be recharged using a generator or an alternator drivingly engaged with the engine, for example. In this way it is ensured that enough electric energy is stored in the electric energy storage device for a subsequent start procedure. Of course, the method may alternatively be initiated by an operator, for example by actuating one or more input devices.